Conventionally, a valve structure forming a check valve that permits forward flow of fluid from an upstream supply circuit to a downstream circuit but prohibits backflow of the fluid from the downstream circuit to the supply circuit has been often used, as has been described in, for example, Japanese Laid-Open Patent Publication Nos. 2002-327706 and 11-315803. Each of these documents describes a hydraulic circuit having a check valve that permits forward flow but prohibits backflow. Particularly, Japanese Laid-Open Patent Publication No. 2002-327706 specifically describes a valve structure forming a typical check valve, which has been often employed in the conventional art, referring to FIG. 3. The valve structure (a load check valve 42) is incorporated in a hydraulic circuit provided in a forklift, as arranged between a pump port defining a part of a supply circuit and a downstream circuit leading to a lift cylinder. When a valve body separates from a valve seat, hydraulic pressure is supplied from the supply circuit to the downstream circuit. When the valve body is received by the valve seat, the valve body prevents backflow from the downstream circuit to the supply circuit.
In this valve structure, when the pressure of the hydraulic fluid in the supply circuit is relatively high, the pressure acts to separate the valve body from the valve seat. In contrast, when the pressure of the hydraulic fluid in the downstream circuit is relatively high, the pressure acts to bring the valve body in contact with the valve seat, thus blocking the corresponding fluid passage. In other words, the check valve of this valve structure is formed simply by arranging the valve seat and the valve body between the supply circuit and the downstream circuit. The valve body is allowed to selectively contact the valve seat or separate from the valve seat, thus switching the corresponding passage between a connected state and a blocked state. This restricts the shape of the passage between the supply circuit and the downstream circuit and the dimensions of the valve seat and the valve body, which are formed in correspondence with the shape of the aforementioned passage. Therefore, such dimensions and shape cannot be largely modified even if such modification is required for reducing pressure loss caused by fluid passing through the check valve formed by the valve structure. It is thus difficult to decrease the pressure loss effectively.